If you don''t want it clickable, you can set
gridControl1[rowIndex, 3].Clickable = false;
If you don''t want it enabled, you can set
gridControl1[rowIndex, 3].Enabled = false;
There is also a ShowButtons style property that allows you to hide the button, and there is a ReadOnly that would make the cell ReadOnly.
You can see these properties in action in the Grid\Samples\CellTypes\ComboboxCells sample.
If it is a GridDataBoundGrid, you cannot explicitly set such properties on a cell by cell basis. (You can set them for the whole column using GridBoundColUmn.StyleInfo fo the column.) If you need to do it on a cell by cell basis, you will have to use PrepareViewStyleInfo as shown in this KB, http://www.syncfusion.com/Support/article.aspx?id=560.